PARENT SESSION
Posters P4Ab Type II reaction centres: Acceptor side. Abstracts (272-288)

The specific rate of Q_A reduction increases as a function of the initial Q_A^- concentration. Szilvia Toth^*,1, Reto Strasser¹, ¹ Laboratory of Bioenergetics, Geneva, Switzerland

ABSTRACT- Upon strong illumination of a dark-adapted photosynthetic sample, the chlorophyll-a fluorescence emission exhibits a polyphasic rise from Fo to Fm, through the steps Fj and Fi during the closure of the reaction centers. The Fo-Fj phase of the fluorescence transient is related to the photochemical processes of PSII. In the presence of DCMU the reoxidation of QA- to QA is inhibited or highly slowed down and the maximal fluorescence is reached after 2 ms (Fj); the total QA reduction can be estimated by the area between the fluorescence transient and the level for the maximum fluorescence (Fm). In this work, pea leaves were treated with DCMU (or left untreated) and measurements of chlorophyll-a fluorescence were carried out using a HandyPEA. Different ratios of reduced and oxidized QA were obtained at the onset of illumination by changing the time intervals between the first and the second measurements (from 0.1s to 1 h). The results show an increase of the specific rate of QA reduction (dQA-/dt per QA-) with increasing initial concentration of QA-. The same phenomenon was observed without DCMU, when a double-normalisation was made between Fo and Fj and the area growth was calculated until Fj. The results may be explained by taking into consideration the PSII antenna heterogeneity or by assuming a cyclic electron transport in PSII, from QA- to the donor side of the reaction center.

KEY WORDS: specific rate of QA reduction, antenna heterogeneity, OJIP fluorescence rise, cylic electron transport in PSII